Hi everyone, thank you to everyone who has made this forum such an awesome source of information. 
In planning college visits, we can only have a limited number during the fall based on scheduling constraints, so the rest will necessarily have to be this summer. I would appreciate people’s thoughts as to which are the most critical to visit during fall when school is in session, versus in the summer. (I searched the forum and didn’t see this question already answered, although maybe I missed it.) The question is in terms of getting the most accurate read on the college’s culture and academics to access fit, learning what is needed about the school to write good application essays, and so forth.
I have a (excessively broad) list below of some schools of interest (with a focus on engineering programs), but please feel free to comment on any other schools that you feel strongly about and why.
Thanks in advance!
UC Berkeley
Brown
Caltech
CMU
Cornell
Duke
Georgia Tech
Harvard
Harvey Mudd
MIT
Northwestern
Princeton
Rice
RPI
Stanford
UC Davis
UCLA
University of Texas
UPenn
Washington University at Saint Louis
Yale
As a parent who has been through this before, my advice would be find your safeties first, as those are the most important schools to know and love. Its easy to fall in love with the reach schools, so those are not necessary to see early in the cycle. What is the student’s in-state flagship? This list above includes several public CA schools, as well as Texas and Georgia – which can all be reaches for out of state students.
That being said, big schools with active summer sessions are still very vibrant places in the summer, so not much downside seeing them then as opposed to school year.
In my opinion, it is important to visit any school while it is session; in addition to the reasons that you state, I think that visiting while a school is in session, with actual students on campus, helps a prospective student to visualize whether he or she can be a part of that student body. (The worst college visits I have been on have been those when no students were on campus – either in the summer, or during a school holiday.) The large public universities are the ones most likely to have actual students on campus during the summer; so I would choose from the list of those schools for summer visits. For Fall visitations, I would (as Midwestmomofboys suggests above) focus on the ones where you are more likely to be admitted; should you/your child be admitted to a reach school, you can always visit in the Spring on an admitted student visitation day and get an idea of who your potential classmates might be.
With respect to your list, while I don’t have your statistics available, you seem to have too many schools that are likely to be reaches or low matches. I might target some more high matches or safeties for visits.
I agree with the poster above me. Visit all of your schools anytime other than the summer. Whether that be in the Fall or in the Spring after you apply.
We saw very few schools while they were in session because my kids were just too busy with school themselves and most tours required them to miss school. We did fine with summer and winter break visits. Yes, I know it’s probably better when school is in session, but I don’t think it’s very realistic. If you must, revisit the front runners when it comes close to decision time.
To answer the OP, Harvey Mudd would be a good choice. We visited there during the summer and there were lots of kids still on campus doing research. And the tour guide stopped to talk to the students about what they were working on—even professors chatted with the tour group.
If you had small liberal arts colleges on your list, I would say “don’t visit during the summer unless absolutely necessary” because they tend to be deserted. But most of your schools are large enough that there will be some students on campus either grad students (or sometimes undergrads) doing research over the summer or high school kids doing summer programs. In the ideal world, you would see the schools while they are in session but we are in the same boat and while we managed to visit 12 schools in the spring, my daughter wants to see 5 or 6 more and we’ll have to do it in the summer because her fall schedule will only permit one or two weekends away and she is taking too many rigorous classes to miss any school. Last summer we visited Dartmouth and Brown and there were plenty of students on campus. If you are going with parents and there are multiple tour guides, I suggest you split up so that you each get a different guide or guides. Your real problem is that your schools are all over the place geographically so you need to get organized.
Thank you everyone for your replies so far!
On the question of visiting safeties/high matches, we are blessed that there are several excellent options in the region that we have already looked at.
So to tally what I have read up to this point,
UC Berkeley
Brown
Georgia Tech
Harvey Mudd
UC Davis
UCLA
University of Texas
have been mentioned as ok or good for a summer visit.
uesmomof2 - thanks; also, 12 visits in the spring is impressive!
ClaremontMom - thanks on Harvey Mudd, good to know
gandalf78, invader71 - agreed; I sure wish they could all be fall visits. Good point on large public universities
Midwestmomofboys - thanks as well; looks like I need to do more research on where there are summer classes in session.